WASHINGTON -- When Mark Covington drives his 1992 Chevy Cavalier at night, the beam of the left headlight wobbles across the road, and in daylight, the red paint on the hood shows an odd ridge on the left side.

He blames his insurance company: After his wife crashed the car into the back of a pickup in April, he said, he took the car to one of the best body shops in Laurel, Miss., but the shop had to use generic parts.

"State Farm just absolutely refused to put GM parts on it," Covington said. Instead, the insurance company insisted on "imitation parts."

"They said that by using those, it would restore our vehicle to the original condition and value, which it did not do," Covington said.

Lawyers are seeking to add him to a large class-action suit against State Farm. But the suit has alarmed consumer advocates, including Public Citizen, which Ralph Nader founded as part of his auto-safety crusades, and the attorneys general of New York, Massachusetts, Pennsylvania and Nevada, along with the National Association of State Insurance Commissioners. They have all filed briefs at the Supreme Court supporting State Farm and opposing people like Covington.

The court could announce as early as Monday whether it will hear arguments in the case.

Plaintiffs say the case involves 5 million drivers and could cost State Farm $2 billion, but their opponents say it would drive up insurance rates nationwide.

Lawyers who filed the class-action suit quote a study done for General Motors that says that cars repaired with generic parts lose $1,670 in resale value, and that some generic parts can be dangerous. Insurance companies dispute both ideas.

Both sides agree that if the class-action suit succeeds, it will affect nearly all drivers around the country. It could also overturn insurance regulations or state laws in New York, Massachusetts and Hawaii, among other places.

According to consumer advocates and the insurance companies, what State Farm calls "quality replacement parts," and others call "after-market parts" meet the same standards for fit and finish, corrosion resistance, strength and durability as the ones produced by GM, Ford or Toyota. The after-market parts have broken a longstanding monopoly of the auto companies on replacement fenders, hoods, bumpers and other parts, consumer advocates say.

The insurance companies say they have a good reason for demanding generic replacements. "They are one of the factors that contributes to our being able to keep our premiums as low as we can," said David Hurst, a spokesman for State Farm. When an insured driver has an accident, everyone is better off if replacement parts cost less, he said.

And manufacturers' parts are expensive. The Alliance of American Insurers calculated that building a 1996 Chevy Lumina from GM parts would cost $72,600, more than triple the sticker price of the car. GM, on the other hand, has advertised that generic parts are not as durable and can cause safety problems.

The dispute is one of the longest-running in the car business.

"I've never seen two industries more dependent on each other that hate each other more," said Jack Gillis, chief spokesman at the Consumer Federation of America, referring to insurers and collision-repair companies. "There's no respect, no trust, no nothing."

Part of the animosity, he said, comes from repair shops' billing insurance companies for original manufacturer parts and then using the cheaper generic ones, a practice that most consumers could not discern. The body shops sometimes do this to spare car owners the cost of their deductibles, or sometimes they do it so they can pocket the difference.

Gillis, who is also the author of an annual buying guide called The Car Book, is not completely neutral; he is also the executive director of a nonprofit organization established by the insurance industry, the Certified Auto Parts Association, to evaluate generic parts.

Gillis said parts that carried his organization's seal were just as good as parts from GM and other manufacturers, a contention that will be an issue in the class-action trial. But he added that only about 3 percent of the parts used in collision repairs were certified by his group. Five percent, he said, come from junkyards, 12 percent are noncertified generic parts, and 80 percent come from auto manufacturers.

The class-action lawyers, who brought suit in Marion, Ill., in July 1997, say the use of generic parts constitutes fraud under Illinois law. "They breach their contract every time they put these flimsy, imitation parts on a vehicle," said Don Barrett, one of the class-action lawyers, in Lexington, Miss.

But many consumer advocates disagree. "All things being equal, the generic product is going to be cheaper," said Brian Wolfman, a lawyer at Public Citizen, which filed a brief in the case. "I'm not suggesting all things are always equal, but sometimes they are equal. You ought not create barriers to introduction of these generic products."

Such products are not only cheaper than the ones made by car companies, but the competition helps drive down the cost of car-company parts, too, consumer advocates say.

The insurance commissioners and attorneys general see another issue, which State Farm asked the Supreme Court to review: Should a court in Illinois be allowed to make what amounts to a national rule on insurance? (The class includes drivers in all states but Arkansas and Tennessee, where separate class-action suits have been filed.)

Insurance regulations in New York encourage the use of generic parts when equal-quality parts are available; a law in Massachusetts requires their use. Hawaii, according to documents in the case, permits the use of manufacturers' parts only if the car owner pays the difference.

Opponents of generic parts cite safety. In an incident that the class-action lawyers intend to introduce in the trial, which is scheduled to begin next April unless the Supreme Court intervenes, Daniel Della Rova was driving his 1988 Honda Accord on a four-lane state highway near Reading, Pa., to his job as an electronics engineer at Lucent Technologies on Jan. 2. He pulled into the left lane to pass a truck, when his hood slammed into his windshield.

The hood blocked the view to the front completely, he said, leaving him to navigate by sticking his head out the window.

Della Rova had bought the Honda used. The car's original owner, Della Rova said, explained that when the Accord was new, he had accidentally backed into it with his other vehicle, a truck, puncturing the hood. The first owner repaired the car with a generic part.

An auto body expert, Charles Barone, who heard about the accident and later retrieved the Honda from a junkyard, said that on the after-market hood, the "striker," the piece of metal that descends and hooks into the latch on the car, was spot-welded inadequately. One weld showed signs of rust, he said. On Honda hoods, he said, the striker is a sturdy metal shaft that passes through several small holes and is then smashed into a mushroom shape, so it cannot come loose.

Barone, a former body shop owner whom the class-action lawyers expect to call as a witness, said, "We all know what junk these parts are. They are approximations at best."

Many insurance companies specify generic parts.

At Allstate, the second-largest auto insurer, Raleigh Floyd, a spokesman, said his company recommended them but would pay for original manufacturer parts if the policy holder wanted.

"The whole reason for going into generic parts is it reduces cost, but it also comes down to peace of mind," he said. Cars can be restored to what the industry calls "pre-loss condition" with generic parts, he said, but in the car owner's mind, "that may mean the part came out of a Toyota plant."

Reprinted from the New York Times.